1. Field of the Invention
The present invention is directed to providing low frequency communications using ionospheric modification. In particular, the present invention provides a method and apparatus for causing interruptions in the ionospheric electrojet to produce ULF/ELF/VLF signals.
2. Related Art
Low frequency communication systems can use the long propagation paths of low frequency waves (on the order of 1 Hz to 1 kHz) inside the earth-ionospheric wave guide to convey information. For example, ELF wavelengths are comparable to the radius of the earth, and horizontal propagation of ELF waves is equivalent to the propagation of a series of earth-ionosphere eigenmodes which have very low spatial attenuation. ELF waves also feature very low absorption in sea water. A 100Hz wave can typically penetrate 100 meters below the ocean surface. As a result, ELF waves have a practical utility in allowing communication with submerged submarines. ELF waves can also penetrate the earth's surface and be used in geophysical exploration.
The conventional approach to ELF wave excitation, as pursued in the late 1960's and early 1970's, employed large ground based antennas, typically on the order of 10.sup.4 km.sup.2. The ground based antenna approach suffers several drawbacks, including the large physical area required for the antenna array, the expense of constructing and maintaining a large facility, and relatively low efficiency. For example, one Navy ELF transmitter uses over 2 MW of ground power to produce only 2W of radiated power, resulting in an efficiency of only 10.sup.-6. Other alternatives which have been proposed include ELF generation by satellite borne antennas and the use of high frequency ground transmitters using the ionosphere as an active medium.
Generating ELF waves by utilizing the ionosphere as an active medium is of particular interest. This technique provides frequency agility and avoids many of the economic drawbacks associated with large and inefficient ground based facilities. Generation of ELF waves by modulating ionospheric currents has been confirmed for both the equatorial and the auroral electrojet in experiments conducted in the USSR (Migulin and Gurevich 1985, Belyaev et al. 1987), the MAX-Planck Tromso facility in Norway (Stubbe et al. 1982, Barr and Stubbe 1984, James et al. 1988), and in the United States (Ferraro et al. 1982, Ferraro 1988, Ferraro et al. 1988).
Electrojet current modulation can be accomplished by using a high power ground transmitter which heats ionospheric electrons locally, to enhance the electron-neutron collision rate. This yields a modified ionospheric region with plasma conductivities (Hall, Pedersen and Parallel) substantially different than the surrounding region which impedes the electroject and induces a local current perturbation. If the heating process is carried out in an intermittent manner with a pulse period in the ELF range, the current perturbation around the heated region will radiate at the ELF frequency in a manner similar to an oscillating dipole. Thus, a "virtual" antenna is created inside the ionosphere. This virtual antenna radiates at the expense of the free energy of the natural electrojet current system.
Most of the experiments performed in this area have used HF frequencies in the range of 2-5 MHz, while the power density at the interaction region varied from between 10.sup.-4 -10.sup.-3 W/m.sup.2. The most exhaustive studies were performed at the Max-Planck Tromso facility The results here were generally consistent with results produced at other facilities, although the precise values of the detected field amplitudes depend on local conditions, characteristics of the HF facility and other specific factors While proving the underlying fundamental principles were sound, experiments to date indicated that the efficiency of such a system would be limited to about 10.sup.-8, or less than that of established ground based systems, such as the Navy system mentioned above. As further discussed below, these previous attempts to utilize electrojet antennas failed to match ionospheric plasma response to antenna operation, resulting in such low efficiencies.